ACCESS KNOB: Parametric Design of 3D-Printable Accessible Synthesizer Controls

Anonymized Author(s)
Double-Blind Review Submission  •
ASSETS '26
The 28th International ACM SIGACCESS Conference
Athens, Greece  •  October 26–28, 2026
Introduction & Co-Design Context

Commercial synthesizer interfaces present significant accessibility barriers for musicians with motor and sensory impairments. Standard controls are typically small, smooth, and closely spaced, requiring high-precision pinch grips and force. Globally, roughly 16% of the population lives with physical disabilities affecting daily functioning and motor coordination [1]. Among clinical cohorts, diabetic peripheral neuropathy eventually affects nearly 50% of adults with diabetes in their lifetime, causing progressive muscle weakness, paresthesia, and diminished tactile sensation in the hands and feet [2].

"Dialysis-dependent kidney failure caused severe hand neuropathy in a retired IT worker and synth collector, leaving his collection of 20 hardware synthesizers completely unplayable. Workarounds like T-wrenches or palm-dragging were fatiguing and lacked expressive control."

This lived co-design encounter inspired **ACCESS KNOB**, an open-source browser-based parametric customizer. Built on Universal Music Design (UMD) principles, the project shifts the paradigm from mass-produced controls to personalized, user-fabricated interfaces that enhance musical expression, independence, and creative control.

References:
[1] World Health Organization. 2022. Global report on health equity for persons with disabilities. Geneva. who.int/health-equity-report
[2] Hicks CW, Selvin E. 2019. Epidemiology of Peripheral Neuropathy and Lower Extremity Disease in Diabetes. Curr Diab Rep 19, 10: 86. ncbi.nlm.nih.gov/articles/PMC6755905
Procedural WASM CAD Kernel

Traditional CAD tools present steep learning curves and lack accessibility. ACCESS KNOB utilizes a client-side WebAssembly-compiled **Manifold** geometry kernel to perform boolean operations in milliseconds, enabling real-time parameter tweaking directly in the browser.

Two Mounting Modes & Three Spindle Bores:

  • Swap-In Mode: Replaces the original manufacturer knob entirely. Generates custom internal flat/splined bores to slide flush onto the bare metal/plastic spindle.
  • Slide-Over Mode: An adaptive sleeve that slides snugly over the original control cap, preserving the instrument's resale value and avoiding complex hardware disassembly.
  • Spindle Bore Profiles: Generates custom flats/splines for D-Shafts, Knurled/Splined (18/24-tooth gears), and Solid Round spindles (secured via integrated M3 set-screw).
SWAP-IN MODE Direct Shaft Replacement SLIDE-OVER SLEEVE Non-Destructive Friction Fit
Dense layout of small custom knobs secured with M3 screws on a synthesizer panel.
Figure 4: Small, high-density configurations showing how varied diameters and shape options fit compact Eurorack or desktop synthesizer panels, utilizing M3 set-screws for secure shaft locking.
2D Vector SVG Stacking Exporter

To democratize fabrication for makerspaces without access to 3D printers, ACCESS KNOB includes a 2D vector SVG stacking exporter. This tool enables subtractive manufacturing (laser-cutting or CNC routing) using sheet materials (wood, acrylic, basswood, or composites).

"The exporter dynamically slices the 3D model into layer profiles, compensates for laser kerf, and adds kerf-compensated slot paths and stacking rods."

System Assembly Mechanics:

  • Kerf Compensation: Sub-millimeter adjustment (default 0.1 mm) ensures slots and outer boundaries match target shapes after laser-cutting.
  • Dual Alignment Rods: Generates two rods of width thickness and length heightKnob + kerf. Slits are cut through all stacked layers, allowing the rods to slide through, aligning the pieces perfectly for gluing.
  • Layer Taper: Outer contours are calculated iteratively at height intervals to preserve the overall tapered ergonomic profile.
Layer 3 (Top) Layer 2 (Middle) Layer 1 (Bore Lock) Alignment Rods Thickness (t) Exploded stacking sequence: Basswood sheet layers aligned by matching rods
Tactile Material Dynamics

Material choice impacts physical sensation, thermal stability, and control responsiveness during musical play:

Material Process Rigidity Friction Tactile Feedback & Damping
PLA 3D Print Medium Medium Hard contact; crisp edges; poor damping
PETG 3D Print High Medium Tough structure; hard contact; poor damping
TPU 3D Print Low High Soft elastomeric grip; high damping
Basswood Laser-Cut Medium Medium-High Warm organic feel; good damping
Acrylic Laser-Cut High Low Hard polished surface; poor damping
Foam Laser-Cut Very Low Low Spongy texture; excellent vibration absorption
Cardboard Laser-Cut Low Medium-High Matte texture; high damping; low durability
Cork Laser-Cut Low-Medium High Textured grip; excellent damping and shock absorption
Geometrical tactile shape profiles generated by the engine.
Figure 1: Procedural geometric profile options (triangle, square, hexagon, octagon, star, and gear) for tactile shape-coding, aiding blind or low-vision users in distinguishing knob functions by touch.
Mechanical Evaluation

To evaluate how parametric adjustments reduce required physical force, we model the peripheral finger force F required to rotate a potentiometer with torque resistance T:

T = F · (D / 2)  ⇒  F = 2T / D

Increasing the outer diameter D from a standard 10 mm to an accessible 32 mm or 40 mm reduces the required finger pinch force by 68.8% to 75.0%, compensating directly for reduced grip strength.

Diameter (D) Required Force (F) Force Reduction
10 mm (Standard) F10 = 2T / 10 0.0% (Baseline)
15 mm 0.67 · F10 33.3%
25 mm (Medium) 0.40 · F10 60.0%
32 mm (Large) 0.31 · F10 68.8%
40 mm (Accessible) 0.25 · F10 75.0%

Grip Shear Force: For musicians utilizing palm-dragging or side-of-hand shear techniques, the functional friction force Ff driving rotation is modeled as:

Ff = μ · Fn    •    A = π · D · H

where μ is the material's friction coefficient, and Fn is the normal force. Taller knobs expand contact area A, distributing downward pressure to enable actuation with minimal pinch grasp requirements.

Star-pattern custom knobs on a synthesizer, showing tactile flutes and grooves.
Figure 3: Star-pattern flutes and deep grooves on custom ACCESS KNOBs. The pronounced edges allow palm-dragging and side-of-hand shear techniques, dramatically increasing rotational torque without requiring a tight pinch.
Accessibility & Future Work

Hands-Free Design Customization:

The web customizer features high-contrast glowing visual focus states, supporting commodity webcam head-tracking and facial gesture interfaces (Google Project Gameface, Apple Voice/Switch Control).

ACCESSIBLE INPUT Face Head-Tracking Voice Command Input Switch Control Scans WASM KERNEL Manifold 3D Engine (Instant CSG Booleans) MULTIMODAL FEEDBACK Aural Sonification (Pitch) Haptic Vibration Cues Aria-Live Speech Updates

Future Work & Clinical Evaluation:

We are expanding the customizer to generate fader caps and drum pads. We propose an IRB-approved protocol with N = 12 musicians with hand impairments to assess custom slide-over vs. swap-in controls. Evaluation metrics will include task completion times, error rates (such as accidental adjacent knob nudging), and System Usability Scale (SUS) scores to evaluate creative agency.

ACCESS KNOB interface showing generative mutations.
Figure 2: Procedural customizer interface showing a batch of 12 generated variants, the real-time WebGL view, and the parameter sliders.